Electromagnetic interference/radio frequency interference conducting strip

ABSTRACT

An aluminum foil strip is provided with adhesive strips on one face surrounding the strip of foil. The strip of foil is then positionable and attachable to an electronic cabinet frame over exposed raw metal areas for intimate electrical contact when the cabinet door is closed. This strip of foil provides electrical continuity between the frame and door as well as a seal against EMI/RFI at the cabinet/door interface.

FIELD OF THE INVENTION

This invention relates to techniques for electrically interconnectingthe movable portions of an electronic systems cabinet or container withthe fixed portions of the cabinet or container and more specifically, tothe material and device used for electrically interconnecting of the twoportions of the cabinet.

BACKGROUND OF THE INVENTION

High performance electronic equipment such as computers and particularlythe processor portion of a computer must be shielded from radiofrequency interference and from electromagnetic interference in order toreliably operate. Electromagnetic interference/radio frequencyinterference (EMI/RFI) interferes with the signals on the differentlines leading to and from the computer and within the computer processoritself, since the lines leading to and from the computer act as antennasand an electrical current may be generated in those electricalconductors as a result of being a receiver for EMI/RFI.

In order to protect the electronic device, from the EMI/RFI and toimprove its performance, the electronic device is typically enclosedwithin a metal cabinet which in turn is grounded. The metal cabinet actsas a receiver for the EMI/RFI and the resulting signals generated in thecabinet are then grounded and thereby prevented from effecting thereliable and efficient operation of the electronic system containedtherein.

Additionally, the electronics of the electronic system generate EMI/RFIinterference. There exists requirements for an EMI/RFI generating deviceto be equipped with shielding to prevent the propagation of suchinterference.

Typically, cabinets are a self-contained item having five sidesincluding a bottom, three sides and a top and additionally a door orclosure attached on the open side. The top and bottom may contain aperforated sheet to allow for air flow in and out of the cabinet. Thecabinet may have a door or closure adjacent to each other, on oppositesides or wherever access is needed to the cabinet.

In order to prevent leakage, either incoming or outgoing, of EMI/RFIthrough the gap between the cabinet enclosure and the door, manyattempts have been made to conductively connect the door and the cabinetand at the same time seal the gap between the two metal pieces. In orderto effectively shield the contents of the cabinet from EMI/RFI, thedoor, in addition to being connected through the hinges, must beefficiently connected electrically to the cabinet itself around theopening to the cabinet. When a large system computer cabinet frame isassembled, for example, it is painted with a conductive paint to providea conductive path to ground and also to provide corrosion protection.

Tin plated copper strips with a silver filled conductive pressuresensitive adhesive and a protective strip that may be removed from theadhesive to allow its installation are commercially available. Thesestrips are applied over the bare metal of the frame, typically before,painting. The conductive pressure sensitive adhesive material carried bythe tin/copper strips relies upon silver fill material in the conductivepressure sensitive adhesive for conductivity between the frame and thetin plated copper strip. The conductivity of the adhesive can vary withthe amount of pressure applied to the adhesive during application.

The cost of this material is exceptionally high due to the silvercontent. The strip is typically applied to the bare metal of the frameand positioned around the periphery of the opening to the cabinet suchthat when the door of the cabinet closes and a resilient electricallyconductive gasket material carried by the door is engaged inface-to-face contact with the tin plated copper strip, the electricalpath between the door through the electrically conductive gasket, thetin plated copper foil and the conductive pressure sensitive adhesive iscompleted to the bare metal of the frame.

An example of such an electrically conductive tape is found in the IBMTechnical Disclosure Bulletin, Vol. 14, No. 2, Jul., 1971, page 474.

A further example of efforts to address the problem of electromagneticcompatibility, i.e., shielding for EMI, is described and disclosed inthe IBM Technical Disclosure Bulletin, Vol. 14, No. 2, Jul., 1971, page518.

U.S. Pat. No. 4,977,296 to Hemming, discloses a metal foil with adhesivestrips placed parallel to each other along the surface of the metal foilthus causing a ridge/valley contour forming a multiplicity of small waveguides which are effective in attenuating electromagnetic energy byvirtue of their physical characteristics. The shielding tape disclosedby Hemming is utilized to join together adjacent panels of shieldingmaterial which have been positioned on a fixed wall or structure.

It is an aspect of this invention to be able to establish electricalcontinuity between the closure of an electronic systems cabinet and theelectronic systems cabinet itself.

It is a further aspect of the invention that the continuity between theclosure and the frame of the cabinet is not dependent upon a conductiveadhesive material, but rather upon highly conductive metal foils.

SUMMARY OF THE INVENTION

An electrically conductive foil of a highly conductive metal is providedwith strips of adhesive material or beads of adhesive material inregions proximate the edges of the foil. These beads or strips may thenbe adhesively engaged, by force or pressure, with the frame of thecabinet which will contain the electronic system or computer. The frameof the cabinet when fabricated and painted, is provided with an exposedraw metal region completely surrounding the opening to the cabinet. Thefoil and its adhesive are positioned such that one face of the exposedfoil, the exposed foil being between the adhesive regions, is positioneddirectly over the raw metal of the frame which was intentionally leftunpainted.

The adhesive acts purely as a holding element to maintain the foilproperly positioned in overlying face-to-face relationship with theexposed raw metal region. The foil will be forced into intimateface-to-face contact and thus establish electrical conductivity with theframe when the cabinet door is closed. The cabinet door carries, inelectrical continuity with the cabinet material, a gasket which isfabricated of conductive material. For example, a highly carbon ormetallically loaded rubber material may be used as such a gasket. Othersynthetic materials may also be used to carry the carbon or metallicparticle loading. When the door of the cabinet is closed, the gasketwill forcibly engage itself, under compressive forces, with the exposedsurface of the foil while at the same time forcing the inner surface ofthe foil against the raw metal surface of the cabinet frame.

The compressive forces of the gasket onto the foil will establishelectrical conductivity between the door or closure of the cabinet andthe frame of the cabinet. At the same time, since the foil will extendcompletely around the opening and will be engaged and compressed betweenthe gasket and the frame at all points, an effective EMI/RFI seal isestablished.

It can be seen that the shortcomings of the prior art are overcome andthe specific aspects of the invention accomplished by the conductivestrip and adhesive material described in summary form above.

A better understanding of the invention may be obtained from thedrawings and the detailed description of the preferred embodiment below.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall perspective view of a cabinet suitable for mountingan electronic system therein together with a closure door.

FIG. 2 illustrates, in exploded form, one embodiment of the inventioncomprising adhesive strips applied to the foil to mount the foil on theframe of a cabinet.

FIG. 3 is an alternative embodiment wherein pressure sensitive adhesivematerial is applied proximate the periphery of the tape permitting theplacement of the tape over the exposed metal portions of the frame ofthe cabinet of FIG. 1.

FIG. 4 is an enlarged view of the upper right-hand corner of the frameand opening of the cabinet of FIG. 1 together with the upper left-handcorner of the interior of the closure or door for the cabinet.

FIG. 5 is a sectional view of a door panel frame and a spring clipattached conductive gasket.

Detailed Description of the Preferred Embodiment of the Best Mode forCarrying Out The Invention

Referring now to FIG. 1, cabinet 10 is shown with the cabinet enclosure12 surrounding a frame 18.

Cabinet 10 is suitable for accepting and containing an electronic systemsuch as a computer. The frame 18 is typically formed of steel such assteel tubing. The cabinet enclosure 12 is preferably sheet metal. Themetal sheet of the enclosure 12 is typically painted with a highlyconductive paint, as is the frame 18.

For example, the paint may be sprayed onto all surfaces of the structureand then heated to fuse the paint into a continuous protective coatingand at the same time, due to its conductive nature, provide conductivityon all surfaces of the enclosure 12 and frame 18.

In order to close the cabinet and to protect the contents from EMI/RFI,a door 14 is provided. The door 14 may likewise be fabricated from sheetmetal and may include a frame 20 or, alternatively, the frame 20 may bemade by bending and forming the sheet metal of door 14 to provide theframe member 20.

In any event, the door 14 is movably mounted with respect to thecabinet, preferably by use of hinges 16.

As shown in FIG. 1, the only electrical contact and conductive pathsbetween door 14 and frame 18 is through hinges 16 which, at best, isunreliable, when the door 14 is closed, there still is a remaining gapbetween the frame portion 20 of door 14 and frame 18. Such a gap istypically sufficient to allow undesired quantities of EMI/RFI to enterthe cavity of the cabinet 10 and to interfere with the reliable andproper operation of the computer system mounted therein.

Referring now to FIG. 4, it is a preferred embodiment of the best modefor carrying out the invention, that frame 18 is fabricated from a steelmaterial, preferably square or rectangular cross-section tubular steel,and welded together. After the frame 18 has been fabricated and coveredwith sheet metal 22. The sheet metal 22 is attached to the exterior offrame 18, by any of several conventional ways, such as with screws,bolts or welds.

The sheet metal 22, for example sheet steel, utilized for the exteriorsurfaces 12 must be preferably highly conductive and act as a partialEMI/RFI shield in and of itself. The exterior sheet metal 22 will beelectrically grounded to the frame by conventional attaching techniques,such as bolting, screwing or welding.

The entire structure is typically painted with a conductive paint andmay be painted using a liquid or powder type material which is sprayedonto the surfaces and then heated to a temperature causing it to fuseinto a continuous protective coating on the metal as well as acontinuous electrically conductive shield on the metal. During thepainting step, an unpainted region 26, which is defined by boundaries 24in FIG. 4, is provided by masking that region prior to painting. Maskingcan be accomplished in many different ways, but a preferred method is touse a strip of magnetic rubber material which will magnetically adhereto the frame 18 and prevent the paint from reaching region 26.

Once the painting has occurred and prior to firing, the magnetic stripsmay be removed exposing the raw steel of the frame 18.

The unpainted region 26 then becomes the major contact and electricalcontinuity point for the completion of the EMI/RFI shield.

The cabinet 10 is closed with a door 14 which may be mounted on thecabinet frame 18 by any conventional means such as, for example, hinge16.

The door 14 is made of an electrically conductive sheet metal which maycontain a frame member 20 or preferably may be fabricated by means ofbending and joining the sheet metal of the door 14. This frame 20provides rigidity to the door and at the same time provides a mountingsurface for an electrically conductive gasket 36. Electricallyconductive gasket 36 is attached to the bare metal on the door 14. Thebare raw metal region 38 of the door 14 is left exposed during thepainting process in the same manner that the unpainted region 26 offrame 18 was left exposed. The remainder of the surfaces of the door 14are painted typically with a decorative coating, then fired or fused tocomplete the coating of paint on the door while at the same timepermanently adhering the door 14 and the paint layer. The electricallyconductive gasket 36 may be one of any of several types. Several typesinclude carbon filled compressible rubber strips where the carbonparticles provided the paths of continuity; or alternatively, a metalsphere or particle loading into the rubber matrix in such quantitiesthat particles will be in contact with adjacent particles thus formingconductivity paths through the rubber gasket.

The flexible gasket 36 may be attached to the bare metal 38 with a metalclip, selective bonding to insure the conductive gasket remains incontact with the bare metal in the cover, provide a recess in the coverto press fit the gasket, or other means that provides a conductive pathbetween the gasket and door.

When the door 14 is moved from its open to closed position the cabinetdoor 14 will assume a face-to-face closed position with frame 18.Electrical continuity provided by the hinges 16 in FIG. 4 isinsufficient to reliably connect the door 14 to frame 18 for purposes ofEMI/RFI shielding.

The electrically conductive foil 40 is illustrated in FIG. 2 withpressure sensitive adhesive strips 42 exploded away from the surface.Pressure sensitive adhesive strips 42 are positioned along the edges offoil 40 and may be of the type having a release sheet engaged with theexposed surfaces which may then be subsequently removed when the foil isto be applied.

The foil 40 is a high purity aluminum foil which will oxidize over timebut where the oxidation will be insufficient to create a conductivityproblem and introduce any significant resistance to the electricalsurface between the door 14 and the cabinet frame 18. The foils ispreferably of the type which is specified in ASTM STANDARD DESIGNATIONB373-90. The foil should be dry annealed and 0 tempered.

An alternative embodiment is illustrated in FIG. 3 where the foil 40 hasdeposited thereon, around its periphery, beads of a pressure sensitiveadhesive 44. The pressure sensitive adhesive in the case of FIG. 3 maybe deposited from a dispenser or nozzle in a molten or semi-molten stateand allowed to cool prior to positioning and application of the foil 40to the frame 18. In the case of FIG. 2, the pressure sensitive adhesivemay take the form of a tape 42 such as a double-face adhesive tape 42.The tape 42 need not be conductive nor need the adhesive component to beconductive inasmuch as the foil 40 will be relied upon for electricalcontinuity and the adhesive is only provided for positioning andretention.

Referring again to FIG. 4, foil 40 is shown positioned over theunpainted regions 26 on frame 18. Between the edges of the foil 40 andthe painted portions of frame 18, resides the pressure sensitiveadhesive strip 42 if the embodiment shown in FIG. 2 is utilized, or thebead of pressure sensitive adhesive material 44 in the case where theembodiment of FIG. 3 is utilized. This arrangement with the adhesive 42or 44 between the inner surface of foil 40 and the painted region offrame 18 holds the foil 40 in position over the unpainted region 26 toprovide protection to the raw steel portion 26 of frame 18. The foil 40will retard rust and corrosion of the unpainted region 26.

When door 14 is closed to completely enclose the electronic system orcomputer within cabinet 10, the electrically conductive gasket 36 willbe brought into face-to-face abutting relationship with foil 40 and itsexterior surface. As the door is forced toward frame 18, the compressionof the electrically conductive gasket 36 will provide a force which willdisplace foil 40 until its inner surface is in face-to-face abutmentwith the unpainted region 26 of frame 18. At this point, electricalcontinuity exists between door 14 through conductive gasket 36, foil 40to the frame 18. Accordingly, any EMI/RFI which is received or picked upby door 14 will then be conducted through the metal of door 14 to theconductive gasket 36. Conductive gasket 36, being in intimateface-to-face engagement with foil 40, will then allow the electricalcurrent generated by the EMI/RFI to be conducted directly to the foil 40and then to frame 18 for grounding.

By utilizing a high purity, highly electrically conductive foil 40, animproved interface between the conductive gasket 36 and unpaintedsurface 26 of frame 18 is accomplished. The foil 40, under pressure fromgasket 36, will conform to and engage the unpainted region 26 over avery large area thus reducing electrical resistance. Likewise,engagement of foil 40 with the conductive gasket 36 will be accomplishedover a large area due to the pliability of foil 40 and the resilient andcompressive nature of gasket 36.

While this particular embodiment of the invention has been describedwith respect to a cabinet 10 and a door 14, it is equally applicable forconnecting the face panels or other frame portions of electronicassemblies with the frame 18. Where the frame 18 may be engaged by aportion of the electronic assembly inserted therein, the continuitybetween the outer frame member of the computer or electronic assemblymay be similarly established. The electronic assembly may have attachedto it a conductive gasket material, and the foil as described and shownin FIGS. 2 or 3 may be attached at strategic locations to the bare metalportion of the frame which would have been left unpainted for thispurpose, thereby establishing continuity between the different shieldingelements of the electronic assembly and the frame enclosure, addingfurther protection against EMI/RFI.

The elimination of an adhesive material from the area between the metalfoil 40 and the exposed metal of the frame 18 improves the electricalconductivity between the foil 40 and the frame 18 by eliminating asubstantial mass of material which is not conductive, as was practicedin the prior art. The electrically conductive particles which must beadded to the adhesive material to provide conductivity greatly increasescosts and tends to reduce the adhesion capability of the pressuresensitive adhesive since the area that the pressure sensitive adhesiveengages, either the foil 40 or the frame 18 is reduced by the amount ofphysical contact between the conductive particle loading and either thefoil 40 and the frame 18. Accordingly, adequate adhesion is also securedwith a smaller area of adhesive by utilizing the described approachReferring to FIG. 5, the gasket 50 may be attached to the door 14 bymetal conductive spring clip 52. If the door 14 is fabricated of anextruded metal or sheet metal, a flange 54 may be formed into the door14. The surface of the flange 54 is left unpainted for electricalcontinuity.

The invention described above may be modified or have changes made to itwithin the scope of the invention as defined by the attached claims.

I claim:
 1. An EMI/RFI shielding enclosure for an electronic system,comprising a container having an electrically conductive frame;saidcontainer having at least six sides, five of said sides of saidcontainer all electrically conductive and electrically connected to saidframe, and defining an opening into said container; a closure meanshaving a frame, for closing said opening of a sixth side of saidcontainer, said closure means formed of electrically conductivematerial; at least an exposed region on said frames of said containerand said closure means exposing said electrically conductive framematerial, and disposed for contact upon closing of said closure means; aprotective, electrically conductive foil overlying said exposed regionon one of said frame or said closure means, and engageable with saidexposed region; an electrically conductive compressible gasketinterposed between said container frame and said closure means andpositioned co-extensive with said exposed regions and in electricallyconnected relation to said closure means; whereby said closure meanscompresses said gasket against said foil forcing said foil intointimate, electrical contact with said gasket and one of said exposedregions, establishing electrical continuity between said closure meansand said frame around the periphery of said opening.
 2. The EMI/RFIshielding enclosure of claim 1 wherein pressure sensitive adhesiveformed into strips along at least two edges of said foil.
 3. The EMI/RFIshielding enclosure of claim 1 wherein said foil is attached over saidexposed region by pressure sensitive adhesive formed into strips alongall edges of said foil.
 4. The EMI/RFI shielding enclosure of claim 2wherein said pressure sensitive adhesive comprises a bead of pressuresensitive adhesive material deposited on said foil.
 5. The EMI/RFIshielding enclosure of claim 2 wherein said pressure sensitive adhesivecomprises double face pressure sensitive adhesive tape.
 6. The EMI/RFIshielding enclosure of claim 3 wherein said pressure sensitive adhesivecomprises a bead of pressure sensitive adhesive material deposited onsaid foil.
 7. The EMI/RFI shielding enclosure of claim 3 wherein saidpressure sensitive adhesive comprises double face pressure sensitiveadhesive tape.